Referring to FIG. 1, a description will be first given of the configuration of a conventional receiver, used in a W-CDMA based radio communication system, which does not employ a diversity reception scheme. It should be noted that the receiver illustrated in FIG. 1 has a configuration which is predicated on a direct conversion scheme which directly converts a received radio frequency signal to a baseband signal. Direct conversion based receivers are described in detail, for example, in Japanese Patent No. 3329264, Japanese Patent No. 3479835, and the like.
As illustrated in FIG. 1, in the conventional receiver, a signal received by antenna 1 is amplified by LNA (low-noise amplifier) 2. The signal delivered from LNA 2 is converted to a baseband signal by orthogonal demodulator 4 after unwanted wave components which are the outside of a reception band have been removed by bandpass filter 3. Here, since the direct conversion scheme is assumed, local oscillator 5 generates a frequency which is the same as the carrier frequency of the received signal.
Orthogonal demodulator 4 delivers two baseband signals, i.e., an I-signal which is a component in phase with a local signal, and a Q-signal which is a component orthogonal to the local signal. The I-signal and Q-signal are amplified by variable gain amplifiers (VGA) 6, 7, and converted to digital signals by A/D converters 10, 11 after unwanted wave components which are the outside of a using channel have been removed by low pass filters 8, 9.
Generally, A/D converters 10, 11 used herein are of approximately eight bits taking into account the size of the circuit and operation speed. This results in approximately 50 dB of difference (dynamic range) between a maximal non-distortion input level available to A/D conversion and the quantization noise. Taking into consideration the peak factor of an input signal and the required S/N ratio, A/D converters 10, 11 cannot make use of their overall dynamic ranges. Therefore, the gains of variable gain amplifiers 6, 7 are adjusted such that the input levels to A/D converters 10, 11 present optimal values. Specifically, a target is set at a level approximately 10 dB lower than a maximal non-distortion input level which can be converted by A/D converters 10, 11 (called the “target level”) to adjust the gains of variable gain amplifiers 6, 7.
The I-signal and Q-signal delivered from A/D converters 10, 11, after they have been converted to digital signals, are supplied to a demodulator circuit (Demodulator), not shown, to restore the original signal. Also, the I-signal and Q-signal delivered from A/D converters 10, 11 are supplied to reception level measuring unit (Level Calculation) 12 to calculate average reception level (power) P for each slot in accordance with the following Equation (1):
                    [                  Equation          ⁢                                          ⁢          1                ]                                                            P        =                              1            M                    ·                                    ∑                              i                =                                  K                  +                  1                                                            K                +                M                                      ⁢                          (                                                I                  i                  2                                +                                  Q                  i                  2                                            )                                                          (        1        )            
Suffixes to I, Q in Equation (1) designate sample numbers of the digitally converted I-signal and Q-signal, and K designates a preceding number of the slot. Also, M designates the number of samples in the slot, and K+1 designates a sample number at the beginning point of the slot. The “slot” refers to a unit time for operational processing determined by the W-CDMA scheme. In the W-CDMA scheme, the average reception level is calculated on a slot-by-slot basis, and the gains of variable gain amplifiers 6, 7 are controlled based on that the value.
Average reception level P calculated by reception level measuring unit 12 is supplied to gain setting circuit (Gain Setting) 13. Gain setting circuit (Gain Setting) 13 generates a control signal corresponding to the difference between average reception level P received from reception level measuring unit 12 and the target level to set the gains of variable gain amplifiers (VGA) 6, 7. Known configurations for variable gain amplifiers 6, 7 include changing the gains in response to an input voltage which is the control signal, or discretely controlling the gains in response to a digital signal which is the control signal, or the like. In this way, feedback control is conducted to optimize the input levels to A/D converters 10, 11.
The foregoing description has been given of the configuration of the conventional receiver, used in a W-CDMA based radio communication system and the like, which does not employ a diversity reception scheme.
Incidentally, radio frequencies currently used by W-CDMA based radio communication systems are in a 2-GHz band, and the propagation characteristics deteriorates in rooms and the like, as compared with a 800-MHz band which is used by PDC (Personal Digital Cellular) and other radio communication systems. Accordingly, for purposes of compensating for deteriorated reception performance in a low electric-field range, studies have been made on the employment of a diversity reception scheme in the W-CDMA based radio communication system as well.
In regard to a general diversity receiver, which is not based on use with a W-CDMA scheme, an exemplary configuration thereof is described, for example, in Japanese Patent Laid-Open No. 2002-141844A. Japanese Patent Laid-Open No. 2002-141844A describes a diversity receiver, for which the direct conversion scheme is not used, to measure the received power of each of two branches (receivers) by converting a radio frequency signal to an intermediate frequency signal and by detecting the envelope of the intermediate frequency signal.
When the diversity reception scheme is applied to the W-CDMA based radio communication system, problems as described below will arise if a diversity reception based receiver is designed as illustrated in FIG. 2 by simply using the conventional receiver illustrated in FIG. 1.
FIG. 2 illustrates the configuration of a diversity reception scheme having two branches (receivers) A, B, given as an example, in which the circuit illustrated in FIG. 1 is applied, as is, to branch A and branch B. However, only local oscillator 5 is shared by branches A, B. While each component in FIG. 2 is designated a similar reference numeral to that in FIG. 1, a suffix “a” is added to reference numerals in branch A, and a suffix “b” is added in branch B for distinguishing them from each other.
In such a configuration, the gains of variable gain amplifiers are adjusted independently in each of branches A, B. An adjusting method therefor is completely similar to that used for the circuit illustrated in FIG. 1. However, since branches A, B differ in average reception level, the gain of each variable gain amplifier is adjusted such that an input level to an A/D converter included in a branch to which the variable gain amplifier belongs is equal to the respective target level.
FIG. 3 shows output levels of the variable gain amplifiers in the respective branches in the configuration illustrated in FIG. 2 after the gain adjustment. In the graph of FIG. 3, the vertical axis is in units of decibel (dB). Also, assume herein that branch A has a lower average reception level than branch B. In such an event, since the gains of the variable gain amplifiers are adjusted independently in each of the branches as described above, the output level of each variable gain amplifier converges to the target level, as a result.
Assume herein that branch B has an S/N ratio (Sb/Nb) of 30 dB. Specifically,
                    [                  Equation          ⁢                                          ⁢          2                ]                                                                      [                                    S              b                                      N              b                                ]                =                  30          ⁢                                          ⁢          dB                                    (        2        )            In this event, a noise level is equal at the reception input of each branch, and signal power Sa, Sb after the gain adjustment is:Sa=Sb=Target Level  (3)Therefore, assuming that the average reception level of branch A is lower, for example, by 10 dB than the average reception level of branch B, the S/N ratio (Sa/Na) is calculated by:
                    [                  Equation          ⁢                                          ⁢          3                ]                                                                                  [                                          S                a                                            N                a                                      ]                    dB                =                                                            [                                                      S                    b                                                        N                    b                                                  ]                            dB                        -                          10              ⁢                                                          ⁢              dB                                =                      20            ⁢                                                  ⁢            dB                                              (        4        )            
Accordingly, after gain adjustment, the noise level Na of branch A is 10 dB larger than the noise level Nb of branch B after gain adjustment, and presents ten times as much power when represented in true value.
As output signals from these two branches are combined in a circuit at a subsequent stage, not shown, the respective signals are added in phase with each other, so that total signal power S is calculated by:[Equation 4]S=(√{square root over (Sa)}+√{square root over (Sb)})2=4×Sa=4×Sb  (5)
In regard to noise, since there is no correlation between branch A and branch B, total noise power N is the sum of the power of these branches. Specifically,[Equation 5]N=Na+Nb=10×Nb+Nb=11×Nb  (6)
Accordingly, the S/N ratio of the overall circuit illustrated in FIG. 2 is calculated by:
                    [                  Equation          ⁢                                          ⁢          6                ]                                                                                  [                          S              N                        ]                    dB                =                                            [                                                4                  11                                ×                                                      S                    b                                                        N                    b                                                              ]                        dB                    =                                                    [                                                      S                    b                                                        N                    b                                                  ]                            dB                        -                          4.4              ⁢                                                          ⁢              dB                                                          (        7        )            The S/N ratio is deteriorated by 4.4 dB from the S/N ratio of the signal received only by branch B which presents a higher average reception level.
In this way, when two sets of circuits illustrated in FIG. 1 are provided, and the gains are adjusted independently on a branch-by-branch basis, the S/N ratio is lower than the ratio that results from a reception signal by only one branch output which presents a higher average reception signal. Another disadvantage is that the number of feedback loops for adjusting the gains is the same as number of branches which results in a complicated circuit configuration.